It has previously been proposed in Laviolette U.S. Pat. No. 4,164,904, the disclosure of which is incorporated hereinby reference, to provide a limited range practice projectile in the form of a spinning tubular projectile (STUP). This prior proposal is a. spin stabilized, open ended tube with an internal wedge at the leading end. The area ratio, that is the ratio of the smallest open area in the core of the projectile to the area of the core passage at the leading edge is such that on firing, the flow through the hollow core of the projectile is supersonic, and when the projectile slows to a lower predetermined supersonic speed due to aerodynamic drag, the flow through the core chokes so that the projectile presents the flight configuration of a solid cylinder with a detached bow shock wave. The resultant high drag acts to curtail the flight of the projectile.
Laviolette teaches the use of a full bore, relatively high drag projectile, which is necessary to provide adequate retard to provoke choking at the desired range. The "C62" projectile designed according to Laviolette to simulate the 105 mm armour piercing discarding sabot (APDS) round has a retard of 0.35 m/s/m compared with a retard of 0.10 m/s/m for the combat round. In addition, the Laviolette full bore projectile has a very large gyroscopic stability (Sg) (see Laviolette, column 10), the C62 having an Sg of 60 while the APDS has an Sg value of 1.5. With these high values for retard and gyroscopic stability, the Laviolette projectile inevitably has a high cross wind sensitivity. As noted in Laviolette this yields a round that is not a perfect trajectory march to the APDS round, although it is a "best fit".
Applying Laviolette to the design of a projectile to simulate a low retard round such as the APFSDS, which has a retard of 0.05 m/s/m, it is found that the full bore STUP has a retard that is excessive and prevents an adequate trajectory match. It therefore becomes necessary to use a subcalibre STUP to reduce the retard and provide a better match to the APFSDS trajectory. However, the subcalibre STUP has a very much increased range, far greater than the acceptable maximum for a practice projectile of this type. The reasons for this are evident from a consideration the theoretical discussion and drawing FIG. 7 in Laviolette. For a projectile with even the theoretical minimum 0.66 area ratio, the speed of the projectile must be less than M2 in order to produce choking. With a low retard to provide a closer match to the low retard APFSDS combat projectile, this speed is not reached until the projectile is much further down range than is the case with a full bore, high retard projectile as taught by the Laviolette patent. It is therefore apparent that no adequate compromise can be made between the trajectory match and range limitation characteristics of the STUP projectile, and the concept of aerodynamic choking is prima facie not viable as a range curtailment technique in simulating a low retard combat round such as the APFSDS.